Mobile broadband in radio telecommunications systems is today becoming increasingly important, e.g. for operators running Wideband Code Division Multiple Access (WCDMA) cellular systems. To enhance system capacity and optimize bit rates, new receiver technologies with advanced receivers in radio base stations are currently being introduced in the WCDMA uplink. These advanced receivers are capable of interference cancellation thereby have higher receiving sensitivity to user equipment (UE). Interference cancellation is a signal processing technology that reduces the amount of interference experienced at the decoding step of a receiver. Two such interference cancellation techniques are described briefly below, these being the whitening GRAKE+ receiver and conventional successive interference cancellation. A more advanced version denoted multi-user detection, performs these operations in parallel between several or all users, this creating an enormously complicated joint estimation and detection problem. For this reason true multi user detection is not attempted in the WCDMA uplink.
In the WCDMA uplink all users transmit on the whole frequency band, thereby achieving frequency diversity gains. The transmissions are allocated to a set of codes, known at the receiver. Each user may use one or more codes for the uplink transmission. This is similar to all direct sequence spread spectrum communication systems. During transmission the signal is affected by the channel which is normally modeled as a time varying finite impulse filter with taps being denoted the channel gains.
In WCDMA a so called RAKE receiver first correlates the received signal with each candidate transmitted sequence, using the codes. This may be interpreted in terms of both correlation and matched filtering. In the next step, the result is combined with the estimated channel model using a second correlation/matched filtering/inner product operation. The channel gain corresponding to a certain filter tap is normally denoted a finger of the RAKE receiver. Following these steps symbol detection can take place.
Since all UEs transmit in the whole uplink frequency band simultaneously, the other users create interference that affect the detection performance of each specific user/UE negatively. To counter this effect, conventional interference cancellation (IC) may be used in receivers of the radio base station. This technology aims at reducing the interference experienced of each specific user.
A first problem with today's uplink receiver solutions is that there are not enough receiver resources to provide all users with the most advanced receivers of the radio base station. A first consequence is a desire to schedule less advanced receivers, such as RAKE and GRAKE receivers, and receivers with higher receiving sensitivity, such as GRAKE+ receivers, in a way that optimizes the uplink performance, as measured in e.g. dropped call rate and radio cell throughput.
Another problem is then to find/design the principles provide receivers to optimize the uplink performance and a furthermore problem is associated with defining the means to implement said principles for scheduling between advanced and less advanced receiver types.